Chiral drugs. Stereoisomerism in molecules can occur because the component atoms are arranged in 3-dimensional space rather than being restricted to a linear array or a 2-dimensional plane. It is of importance since the interaction between biologically active sites and substrate molecules, e.g. drug-receptor site interaction, is highly stereoselective. Enantiomers of the same drug can have different pharmacodynamic and pharmacokinetic properties. This may translate into potential health benefits, such as an improved safety margin, if one of the enantiomers has more favorable therapeutic and pharmacokinetics characterictics. The rationale for development of chiral drugs is the suggestion that the single stereoisomer drugs would provide superior therapy by allowing reduction of dosage, reduced variability in metabolism and response, simpler dose-response relationship and improved tolerability. This article reviews the concept of chirality and stereoselective nature of drug action, and its impact on pharmacotherapeutics.